fst951

So here is how it works.

The shift fork is attached to a rod which is actuated when you select a gear combination. The fork then moves a slider which comes out of the neutral position and moves toward a servo-syncro band. The engergizer then takes over and allows the syncro to more properly engage the slider and equalize rpms to avoid clash when the slider actually makes it to the dog teeth or dog drivers. As far as holding in gears, it may well be the stops and locks that are allowing it to back out combined with a worn slider or even syncro. I don't think the syncro is the likely culprit, but I damn well would replace it with the engergizer, lock, stop and check out the fork.

Good luck.

jpitman2

Its also NOT a good idea to hold the shift lever while NOT shifting - constant load on the shifter can lead to excessive wear in the selector forks and detents, eventually leading to problems like the above.
jp 83 Euro S AT 49k

Vilhuer

Few pictures of gearbox internals to show what they look like. This is G28.57 GTS box. Basic layout is same on all 928 manual boxes. '78-84 have different multi part Porsche patented syncro ring design when '85-95 use single piece Borg Warner type. Third reverse gear mounted on separate short shaft is missing from pictures.



Left (thin) end of pinion shaft (at center top) goes inside input shaft (top left). This positions gears correctly for rotating lay shaft (center left).



Two darker mat surface areas is where input shaft roller bearings are in contact with pinion shaft. 5th gear syncro ring is on gear. It just hangs there, nothing holding it in. 4/5 selector ring and assosiated parts are off from input shaft exposing nut that holds together gears in pinion shaft.













One of the weak points of 928 transmission is input shaft. Factory beafed up shaft and ball bearing but didn't completely solve problem of TT center shaft mounting to the input shaft. On relatives '85 G28.10 box one of the three bore pieces is snapped right of. Looks difficult to fix but we dry anyway.

Vilhuer

Side bearings inner race is still on differential. Thinking of clean ways to get it out so that PSD bearing would not suffer too much.





Opening input shaft nut is IMO most difficult task in entire syncro change. That is if there are not correct special tools for the job. Very few places have them.

Older style '78-84 box DO NOT need tool like in these pictures. Nut on those is other side of all gears and looking to WSM it is easier to open. Opening it is not even needed unless pinion bearing needs to be changed. Tool for it certaily looks easier to replicate.

Tools in pictures were custom made locally using other dismantled GTS gearbox parts as quide. Works perfectly and we have already opened nut on both gearboxes we are working on currently. Large ring thing is used to turn input shaft when nut (on the right) is held put on vice by tube type tool. Nuts torque is 300 Nm IIRR. There is only one or two other parts with similar/higher torque in entire 928. So it's difficult to open that nut without correct tools. Almost everything else special needed to rebuild box can be made quite easily it seems.







This is 4th gear of the pinion shaft. When nut is opened these can be taken out by hand. All the rest must be pressed out using reverse gear to push them of pinion shaft.



Syncro ring rides on polished part of the gear helping to engage it when large ring visible in earlier pics is pushed towards gear.







Older type syncro rings are more complex four part items.